A genome scan was conducted in the North American Holstein-Friesian populationfor quantitative trait loci (QTL) affecting production and health traits usingthe granddaughter design. Resource families consisted of 1,068 sons of eightelite sires. Genome coverage was estimated to be 2,551 cM (85%) for 174genotyped markers. Each marker was tested for effects on milk yield, fat yield,protein yield, fat percentage, protein percentage, somatic cell score, andproductive herd life using analysis of variance. Joint analysis of all familiesidentified marker effects on 11 chromosomes that exceeded the genomewide,suggestive, or nominal significance threshold for QTL effects. Large markereffects on fat percentage were found on chromosomes 3 and 14, and multimarkerregression analysis was used to refine the position of these QTL. Half-siblingfamilies from Israeli Holstein dairy herds were used in a daughter design toconfirm the presence of the QTL for fat percentage on chromosome 14. The QTLidentified in this study may be useful for marker-assisted selection and forselection of a refined set of candidate genes affecting these traits.